This is totally correct the word Fully being very important , The order was found in the morning and reached McClellan around midday within the first 3 hours he has dispatched Pleasanton and his cavalry to see if the orders held any merit , At finding they did at around 6.30pm he got Franklins corps in motion.
This is incorrect.
The regiment that found it went in bivouac at midday. Within half an hour or so of going into bivouac Corporal Mitchell spotted the envelope and handed it to Sergeant Bloss. It slowly started making it's way up the chain-of-command. It was handed to the company commander (Capt Kop), who took it to Colonel Colgrove. After examining it Colgrove and Kop took it to brigade HQ, where General Gordon examined it. General Kimball was also present and advised them to hand it up. They took it to General Williams HQ.
Williams was absent from his HQ, but Colonel Pittman of his staff recognised the signature, and sent a runner to ask Williams to return. When he did Williams, Pittman, Kimball, Colgrove, Kop and some other staffers discussed the matter and decided it should be sent on to General McClellan. General Kimball carried it, and went looking for McClellan's HQ.
Now, McClellan's HQ was not established at the Steiner Farm until ca. 1500 hrs. McClellan himself was forward with Cox. They had just gained possession of the Braddock Pass through the Catoctins and McClellan was ordering Cox to march to Middletown beyond in support of the cavalry advancing to seize the South Mountain passes. When McClellan went to his HQ at ca. 1430 the Lost Order had not yet arrived, but it arrived shortly thereafter. McClellan sent a copy forward to Pleasonton timestamped 1500. The written confirmatory orders to Cox for his advanced went out timestamped 1535.
There can of course be no noon telegram (the original is timestamped midnight) because at noon there is no operational telegraph wire, the rebels have downed it. At 1500 hours it is noted that the line has just been put back up across the Monocacy River, connecting Frederick and Monocacy Stations. The line then had to be relaid from Monocacy Station to Urbanna. The first telegram sent was the 2300 to Halleck.
Common perception is McClellan did nothing for 18 hrs and of course that's false but the real question should have been did he do enough within that timeline.
What else could he have done? Once the Braddock Pass was seized McClellan immediately sent 9th Corps across the Catoctins to support Pleasonton's seizure of Turner's and Fox's Gaps. The 9th Corps filled the National Road out of Frederick until ca. 0200 or 0300 hours the next morning. Hooker was under orders to follow immediately, and did exactly that.
There is literally no physical way of moving faster, because there is only one road going through a mountain pass.
The other question of course is if McClellan outfoxed Lee why did he let him escape?, Same I suppose could be said of George Meade at Gettysburg but the difference between the two was Meade fought for 3 days dealing with an army nearly equal to his own , McClellan on the other hand had a 3-1 advantage and Franklins Corp that was in reserve.
McClellan certainly didn't have a 3:1 advantage when correct counting is done. It's certainly less than 2:1 at the extreme, and somewhere between 1:1 and 3:2. Franklin's Corps was in the front lines, if not heavily engaged.
As to Lee's escape, after having moved all his heavy equipment etc. over the river (which he'd already done by the morning of the 17th) the infantry crossed during the night. At dawn McClellan aggressively pursued to the banks of the Potomac where they were greeted by ca. 50 artillery pieces across the river engaging his vanguard.
McClellan was a great Army organizer and gave his men pride unfortunately he was a poor battle commander , I've read some of Frye's account and I'm stunned how he can presume McClellan was anything else but and over cautious poor field commander , Oak Grove was a classic example of McClellan at work and the only other example of a significant offensive move by him apart from Antietam.
Offensive is an interesting term. You misuse it to suggest that only a bayonet charge is an offensive. In fact apart from the covering actions of the Seven Days all of McClellan's actions are on the offensive. Even Seven Pines, where the rebels adopt the tactical offensive, is an offensive movement by McClellan - seizing ground that the enemy has to attack and forcing him to come out of his entrenchments to attack yours.
Hooker and Burnside ended up Corp commanders , Pope was sent to other commands but poor George got nothing after he was sacked for the second time and I wonder why.
He was too senior. He was the most senior general in the US Army until Grant was made Lt-Gen. It was basically impossible to make him a subordinate. Now there was talk of him taking command of the Western Theatre in Halleck's old role, but that came to naught. The radicals had seized the balance of power after the November 1862 elections and they wanted McClellan and several others gone. Lincoln's party before then didn't need his Radical faction to govern and ignored them, but Lincoln's reduced position post the November '62 elections meant he now needed their votes to pass legislation. They extracted their price in a massive purge of "disloyal" officers as they saw it - Buell, McClellan, Porter and Butler were the first to go, and they soon continued their program. There was no military reason for it; it was the price Lincoln paid to maintain a functional government.